[xzyyyz]

There is a lot of harm:

1. In order to get a job, a person need to quit one. If there is a risk (in form of trial period), why to accept such an offer? As result, best candidates pass, or at least stick to existing positions longer. Or pay raise must be more significant.

2. Reputational harm: read about Amazon's hire-to-fire practice. And try to guess, why a lot of people do not consider employment with Amazon.

3. Work harm: any new employee is investment. It has to be onboarded, and it is a heavy load for the accepting team.

Of course, it heavily depends on the job market segment. I know software eng. only. If most candidates are unemployed, trial period could be a blessing. In the current market, why would I even apply (and spend my time on the interview?